Gdk: Transform the out param of Rectangle.Union() to a return value.
Gdk: Transform the out param of Rectangle.Union() to a return value.

Not only this is prettier but it's also aligned with the 2.12.x API.

akrisiun/gtk-sharp,orion75/gtk-sharp,openmedicus/gtk-sharp,sillsdev/gtk-sharp,Gankov/gtk-sharp,antoniusriha/gtk-sharp,openmedicus/gtk-sharp,sillsdev/gtk-sharp,orion75/gtk-sharp,antoniusriha/gtk-sharp,antoniusriha/gtk-sharp,antoniusriha/gtk-sharp,sillsdev/gtk-sharp,orion75/gtk-sharp,akrisiun/gtk-sharp,Gankov/gtk-sharp,Gankov/gtk-sharp,Gankov/gtk-sharp,akrisiun/gtk-sharp,openmedicus/gtk-sharp,akrisiun/gtk-sharp,openmedicus/gtk-sharp,sillsdev/gtk-sharp,openmedicus/gtk-sharp,akrisiun/gtk-sharp,orion75/gtk-sharp,sillsdev/gtk-sharp,Gankov/gtk-sharp,Gankov/gtk-sharp,openmedicus/gtk-sharp,antoniusriha/gtk-sharp,openmedicus/gtk-sharp,orion75/gtk-sharp